Cellulose filaments (CF) previously referred to as cellulose nanofilaments (CNF) are known to have many interesting properties one of which is increasing the dry and wet strength properties of paper when used as an additive in the production thereof. They are produced by multi-pass, high consistency refining of wood or other plant fibers at a high level of specific energy using high consistency refiners (Hua, X., et al. High Aspect Ratio Cellulose Nanofilaments and Method for their Production. U.S. Pat. No. 9,051,684 B2, 2015; PCT/CA2012/000060; WO 2012/097446 A1, 2012). They have superior reinforcement ability over other cellulose micro- or nano-materials such as microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC) prepared using other methods for the mechanical fibrillation of wood pulp fibers. This is because of the longer lengths and higher aspect ratio of CF as a result of the unique multi-pass and high consistency production process which minimizes fiber cutting. Consistency is the weight percentage of a cellulose material in a mixture of the cellulose material and water. High consistency (HC) in fiber processing refers to a consistency typically in the range of 20-65%. High consistency refiners are refiners with a discharge consistency of over 20%.
Production of CF that has superior reinforcement ability typically involves 2-14 passes of the wood or other plant fiber material through a high consistency refiner. The total specific refining energy applied is in the range of 2,000-20,000 kWh/t. One drawback for the production of CF that has superior reinforcement ability is the high energy consumption.
Single-pass, high consistency (35%) refining, followed by single-pass, low consistency (LC) (2%) refining of bleached softwood kraft pulp fibres has been evaluated for the reinforcement of a thermomechanical pulp (TMP)-based furnish (Sjoberg, J. C. and Hoglund, H. Proceedings to International Mechanical Pulping Conference, Minneapolis, Minn., May 2007). Only small (3-12%) improvement in tensile energy absorption (TEA) of the paper made from such HC and then LC refined kraft pulp and the TMP pulp over the paper made from the reference LC refined kraft pulp and the TMP pulp was achieved, even though the total energy applied for the HC and then LC refining (332-398 kWh/t) was significantly higher than that for the reference LC refining (84-89 kWh/t).
A method for the production of microfibrillated cellulose (MFC) fibers using multi-pass low consistency (1-6%) refining of cellulose fibers has been described (Suzuki, M. and Hattori, Y. PCT WO2004/009902; U.S. Pat. No. 7,381,294 B2, 2008). The number average fiber length and the water retention value of the MFC fibers were reported to be 0.2 mm or less and 10 mL/g or more, respectively. The reinforcement ability of the MFC fibers, however, was not shown.
A method for the production of microfibrillated cellulose (MFC) using single-pass high consistency (>15%) refining/pretreatment, followed by multi-pass medium consistency (6-15%) or followed by multi-pass, medium consistency (6-15%) and then multi-pass low consistency (<6%) refining of cellulose fibers has been described (Sabourin, M. and Luukkonen, A. U.S. Pat. No. 8,906,198 B2, 2014). The average fiber length of the MFC produced was also reported to be 0.2 mm or less while the water retention value of the MFC was said to be 20 mL/g or more. The reinforcement ability of the MFC, however, was not shown. Furthermore, the energy applied in the high consistency refining/pretreatment is limited to 600 kWh/t or less in this method.
A method for the production of MFC using low to medium consistency (<12.5%) refining, followed by dewatering and then by medium to high consistency (12.5-20%) refining of pulp fibers has been reported (Heiskanen, I., et al. PCT WO 2014/106684 A1). Although one of the goals of using the reported method was to reduce the energy consumption in MFC production, no energy data for the reported method vs. those for the low to medium consistency refining or for the medium to high consistency refining were presented. It was suggested in this publication that a sequence of refining steps at a constant low consistency would require progressingly increased energy, thus being ineffective, and that high consistency refining without a preliminary/preceding low consistency refining would similarly be energy consuming and ineffective.